What can I expect if I make the switch from my current mallory promaster coil to an msd 6A ignition box.

Probably very little, if any, if the Mallory is set up right.

Probably nothing.

Theory:
There is no such thing as hidden power in adding a trick ignition system. Only LOST power if your ignition fails to light the fires when it should. The whole reason for the hot CD ignition system is because as compression increases, it act's as resistance to the energy jumping the gap at the plug. Race cars with 13 and 14:1+ compression create a significant resistance to the spark jumping the gap. Electricity (voltage) follows the path of least resistance and if it can find a way out of the wire or terminal it WILL! So in high compression motors you have to run a VERY well insulated spark plug cable, make sure your not toasting the boots on the headers, and do your best to keep the energy inside the cable so that the plug is the path of least resistance,,, then all you need is enough energy to force the spark across the gap. So, as long as your plugs are firing with a decent spark kernel, there is really nothing to be gained.

Now the 'reality' part.
A 3 or 4 decade old ignition system is seldom going to provide an 'accurate, consistant spark'. Just as an old HEI coil begins to fall on it's face because there just isn't enough 'soak' time to create the high voltage necessary at high rpm's, the modules, corroded caps and rotor tips, baked spark plug cables, even faulty grounds and wiring that fails to provide battery voltage to the system,,, all sorts of things create the resistance that causes a misfire or a very poor ignition.

Will a ignition box help cure other faulty parts,, NO, but it will act as a bandaid by providing multiple sparks at low rpm's. IMO before you drop the $'s on a ignition box, you need to ask yourself two questions,,
1. Do I need it? If your running a 8:1 motor with a aftermarket intake, small tube heders and a mild street cam that neve sees the high side of 5000rpm,,, It's highly unlikely the box would provide anything more than bringing your existing ignition up in top shape.
2. Is the rest of the ignition 'worthy' of the investiment. Again, a bandaid may hide a small wound, but it's not going to do much for a gushing artery. Adding a box may temporarily hide faults in the rest of the ignition, but usually they end up being overworked and over heated trying to fight the resistance in the system,,,, then the owner wants to blame the 'garbage' box they bought.

There is also a LOT to be said for todays digital ignitions and their accuracy. IMO you don't invest in a MSD6a or 6AL. It just doesn't make sence any more. The Digital Mallory HyfireVI and the MSD Digital 6 have functions built in that are 'usually' a necessity of the motor that requires the box. Start retard, window/rpm switch, low and high side rev limiters,, all stuff you have to buy and add to the other boxes (and they chrge DEARLY for those add-on's)

So have I helped make the decision,, probably not. But there's my 2cents anyway.

Wow, that's a lot of good info. I think I fall into number 2. So my Mallory promaster is a good coil for what I have. It's an upgrade over the HEI I assume.